Hillary Clinton on Wednesday became the first US secretary of state to visit Laos for 57 years, on a trip focused on the damaging legacy of the Vietnam War and a controversial dam project.

Hillary Clinton on Wednesday became the first US secretary of state to visit Laos for 57 years, on a trip focused on the damaging legacy of the Vietnam War and a controversial dam project.

At Vientiane's flag-bedecked Wattay International airport, Clinton was given flowers by girls in traditional purple-silk costumes, kicking off her brief but historic trip.

"It's a pretty big deal for the Laotians, and we will underscore a number of areas that we're working on together," a senior US official said.

These include left over ordnance from the war which ended in 1975 as well as the continuing effects of the defoliant Agent Orange, used by US forces to try to flush out communist forces.

The US wants to push for more access for excavating the remains of those missing in action, the official said, adding that two planned trips had been cancelled in the last 16 months due to resistance from the Laotian side.

Another of the main thrusts of the trip will be talks on controversial plans by Laos to build a massive dam on the Mekong River, which governments and environmentalists warn could have a devastating effect on millions of people.

During regional talks in Bali last year, Clinton called for a moratorium on dam building along the river -- the world's largest inland fishery. Activists say the dam projects could spell disaster for 60 million people who depend on the Mekong waterway.

Clinton is only the second secretary of state to visit Laos after John Foster Dulles, who spent a day in the then-monarchy in 1955. Experts say that all those years ago they had to clear the water buffalo from the Vientiane airport runway so his plane could land.

Clinton was invited to Laos by foreign minister Thongloun Sisoulith in 2010 who was the first top Laotian official to visit Washington since the Soviet-backed communist rebels swept to power, ousting the monarchy, in 1975.

During her four-hour whirlwind trip, which was front page news in Laos this week, she will hold talks with Thongloun as well as with Prime Minister Thongsing Thammavong.

US relations with Laos, while never severed, were long tense, in part over its campaign against the Hmong hill people who assisted US forces during the Vietnam War.

But the United States established normal trade ties with Laos in 2004 and annual US aid to Laos stands at around $21 million, according to the embassy website.

Laos, which is one of the poorest nations in the world and has just 6.5 million people, is looking to boost its international trade.

The landlocked nation has been growing at about seven percent annually in recent years, its government says, and it is aiming for membership of the World Trade Organization.

Human Rights Watch meanwhile pressed Clinton to call for the release of detainees at the Somsanga drug centre, which it claims holds children and the mentally-ill alongside drug addicts.

"The Lao government and the US State Department claim that Somsanga is a modern healthcare centre," said Joe Amon, HRW director in a statement.

"But a decade of US funding hasn't changed the fact that it's a brutal and inhumane detention centre where the Lao government puts 'undesirable' people."